KUALA LUMPUR, MALAYSIA—The search for Malaysia Airlines Flight 370 turned into a criminal investigation Saturday when Malaysia declared the plane had been deliberately diverted from its scheduled route to Beijing and then flown for up to seven hours toward an unknown point far to the west.

Prime Minister Najib Razak’s announcement confirmed days of mounting speculation that the disappearance of the Boeing 777 over a week ago was not accidental, shifting the focus of the investigation to the 239 people aboard — in particular, the pilot and co-pilot.

As Malaysian authorities released a map Saturday afternoon showing that the last satellite signal picked up from the plane put it somewhere along one of two arcs spanning a large expanse of Asia, Najib said he would seek the help of governments in the now much widened search zone.

Police officers were seen Saturday going to the home of the flight’s pilot, Zaharie Ahmad Shah, near Kuala Lumpur. Malaysian news media reported a search had taken place, but the police would only say there would be a news conference Sunday.

A satellite orbiting nearly 36,000 kilometres above the middle of the Indian Ocean received the transmission that, based on the angle from which the plane sent it, put it somewhere along one of two arcs. One runs from the southern border of Kazakhstan in Central Asia to northern Thailand, passing over some hot spots of global insurgency and highly militarized areas. The other runs from near Jakarta to the Indian Ocean, roughly 1,600 kilometres off the west coast of Australia.

Nijab said the plane changed course not long after it took off: “These movements are consistent with deliberate action by someone on the plane.”

(Experts have previously said that whoever disabled the plane’s communication systems and then flew the jet must have had a high degree of technical knowledge and flying experience.)

Nijab said someone aboard the plane disabled one communications system as it flew over the northeast coast of Malaysia. A second system abruptly stopped broadcasting its location, altitude, speed and other information at 1:21 a.m., while the plane was a third of the way across the Gulf of Thailand from Malaysia to Vietnam.

Military radar data showed the plane then turned and flew west across northern Malaysia before arcing out over the wide northern end of the Strait of Malacca, headed for the Indian Ocean.

The flight was scheduled to land at 6:30 a.m. in Beijing so when its last signal came — at 8:11 a.m. — it could have been nearly out of fuel, Najib said.

“Due to the type of satellite data, we are unable to confirm the precise location of the plane when it last made contact with a satellite,” he said, reading a statement in English.

The northern arc described by Najib passes close to northern Iran, through Afghanistan, northern Pakistan, northern India, the Himalayas and Myanmar. The southern arc, from Indonesia to the southern Indian Ocean, covers open water with few islands. If the aircraft took that path, it might have passed near the Cocos (Keeling) Islands, where there is a small airport.

After Najib’s announcement at an airport hotel, China’s Ministry of Foreign Affairs said technical experts would be sent to Malaysia. Two-thirds of the people on the jet were Chinese citizens.

A ministry spokesman, Qin Gang, said China would shift its planes and ships to search areas west of Malaysia, a region that includes India and other nations that have tensions with China.

American investigators cited the need for hard information in the inquiry.

“It doesn’t mean anything. All it is is a theory,” one senior U.S. official said. “Find the plane, find the black boxes and then we can figure out what happened. It has to be based on something, and until they have something more to go on it’s all just theories.”

U.S. investigators have much of the flight data obtained from radar and satellites, but have been given far less information about the pilots and passengers.

Soon after the plane disappeared, FBI agents and other Americans “scrubbed” the names of pilots and passengers — including two Iranian men traveling on stolen passports — to determine whether they had any terrorist connections, but found none, the officials said.

Mikael Robertsson, a co-founder of Flightradar24, a global aviation tracking service, said the way the plane’s communications had been shut down pointed to the involvement of someone with considerable aviation expertise and knowledge of the route, possibly a crew member, willing or unwilling.

The Boeing’s transponder was switched off just as the plane passed from Malaysian to Vietnamese air traffic control space, making it more likely that its silence would not arouse attention, he said by phone from Sweden.

“I think the timing of turning off the signal just after you have left Malaysian air traffic control indicates someone did this on purpose, and he found the perfect moment when he wasn’t in control by Malaysia or Vietnam. He was like in no-man’s country.”

Xu Ke, a former commercial pilot who has advised the Chinese government on aviation security, said the details suggested that at least one crew member was involved in seizing control of the aircraft, either willingly or under pressure.

“The timing of turning off the transponder suggests that this involved someone with knowledge of how to avoid air traffic control without attracting attention,” he said.

“The Boeing 777 is a relatively new and big plane, so it wouldn’t be anyone who could do this, not even someone who has flown smaller passenger planes, even smaller Boeings,” he said.

On Saturday, the announcement from Malaysia brought dismay in Beijing among family members and friends of the many Chinese who were on the missing plane.

For a week, the families and friends have gathered at a hotel, receiving updates from Malaysia Airlines employees and waiting for news. Several managed to find some relief in the announcement that someone had apparently taken control of the plane because that left a faint hope that the passengers were somehow alive somewhere.

A U.S. P-8A Poseidon, the most advanced long-range anti-submarine and anti-surface warfare aircraft in the world, was to arrive over the weekend and sweep parts of the Indian Ocean.

It has a nine-member crew and has advanced surveillance and reconnaissance capabilities, the U.S. Defence Department said in a statement.